1. Field of the Invention
The present invention relates to an electrically-driven liquid crystal lens, and more particularly, to an electrically-driven liquid crystal lens which includes a light shade to be switched on or off according to whether or not voltage is applied thereto, thereby reducing a cell gap of a liquid crystal layer, and a stereoscopic display device using the same.
2. Discussion of the Related Art
At present, services for rapid dissemination of information, to be constructed based on high-speed information communication networks, have developed from a simple “listening and speaking” service, such as current telephones, to a “watching and listening” multimedia type service based on digital terminals used for high-speed processing of characters, voices and images, and are expected to be ultimately developed into hyperspace 3-dimensional stereoscopic information communication services enabling virtual reality and stereoscopic viewing free from the restrains of time and space.
In general, stereoscopic images representing 3-dimensions are realized based on the principle of stereo-vision via the viewer's eyes. However, since the viewer's eyes are spaced apart from each other by about 65 mm, i.e. have a binocular parallax, the left and right eyes perceive slightly different images due to a positional difference between the two eyes. Such an image difference due to the positional difference between the two eyes is called binocular disparity. A 3-dimensional stereoscopic image display device is designed based on binocular disparity, allowing the left eye to view only an image for the left eye and the right eye to view only an image for the right eye.
Specifically, the left and right eyes view different 2-dimensional images, respectively. If the two different images are transmitted to the brain through the retina, the brain accurately combines the images, reproducing depth perception and realism of an original 3-dimensional (3D) image. This ability is conventionally referred to as stereography (stereoscopy), and a display device to which stereoscopy is applied is referred to as a stereoscopic display device.
In the meantime, stereoscopic display devices may be classified based on constituent elements of a lens which realizes 3-dimensional images. In one example, a lens using a liquid crystal layer is referred to as an electrically-driven liquid crystal lens.
Generally, a liquid crystal display device includes two electrodes opposite each other, and a liquid crystal layer interposed between the two electrodes. Liquid crystal molecules of the liquid crystal layer are driven by an electric field created when voltages are applied to the two electrodes. The liquid crystal molecules have polarization and optical anisotropy characteristics. Here, polarization refers to a change in molecular arrangement direction according an electric field, which is caused as electrons in liquid crystal molecules are gathered to opposite sides of the liquid crystal molecules when the liquid crystal molecules are under the influence of an electric field. Also, optical anisotropy refers to a change in path or polarization of light to be emitted according to an incidence direction or polarization of incident light, which is caused by an elongated shape of liquid crystal molecules and the above-mentioned molecular arrangement direction.
Accordingly, the liquid crystal layer has a transmittance difference due to voltages applied to the two electrodes, and is able to display an image by varying the transmittance difference on a per pixel basis.
Recently, there has been proposed an electrically-driven liquid crystal lens in which a liquid crystal layer serves as a lens based on the above-described characteristics of liquid crystal molecules.
Specifically, a lens is designed to control a path of incident light on a per position basis using a difference between a refractive index of a lens constituent material and a refractive index of air. In the electrically-driven liquid crystal lens, if different voltages are applied to electrodes located at different positions of the liquid crystal layer so as to create an electric field required to drive the liquid crystal layer, incident light introduced into the liquid crystal layer undergoes different phase variations on a per position basis, and as a result, the liquid crystal layer is able to control the path of the incident light in the same manner as an actual lens.
Hereinafter, an electrically-driven liquid crystal lens of related art will be described with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating an electrically-driven liquid crystal lens of the related art, and FIG. 2 is a schematic view illustrating a potential distribution of the electrically-driven liquid crystal lens of FIG. 1 after voltage is applied to the electrically-driven liquid crystal lens.
As illustrated in FIG. 1, the electrically-driven liquid crystal lens includes first and second substrates 10 and 20 opposite each other, and a liquid crystal layer 30 formed between the first substrate 10 and the second substrate 20.
First electrodes 11 are arranged on the first substrate 10 and are spaced apart from one another by a first distance. In the two neighboring first electrodes 11, a distance from the center of one first electrode 11 to the center of the other first electrode 11 is referred to as a “pitch”. Repeating the same pitch for the respective first electrodes results in a pattern.
A second electrode 21 is formed over the entire surface of the second substrate 20 opposite the first substrate 10.
The first and second electrodes 11 and 21 are made of transparent metal. The liquid crystal layer 30 is formed in a space between the first electrodes 11 and the second electrode 21. Liquid crystal molecules of the liquid crystal layer 30 have a parabolic potential surface due to a property reacting according to the intensity and distribution of an electric field and thus, have a phase distribution similar to that of the electrically-driven liquid crystal lens as illustrated in FIG. 2.
The above-described electrically-driven liquid crystal lens is realized under the condition that high voltages are applied to the first electrode 11 and the second electrode 21 is grounded. With this voltage condition, a vertical electric field is strongest at the center of the first electrode 11, and the intensity of the vertical electric field decreases away from the first electrode 11. Accordingly, if the liquid crystal molecules of the liquid crystal layer 30 have positive dielectric anisotropy, the liquid crystal molecules are arranged according to the electric field in such a way that the liquid crystal molecules are upright at the center of the first electrode 11 and are gradually tilted approximately horizontally away from the first electrode 11. As a result, in view of light transmission, an optical path is shortened at the center of the first electrode 11, and is lengthened with increasing distance from the first electrode 11, as illustrated in FIG. 2. Representing the length variation of the optical path using a phase surface, the electrically-driven liquid crystal lens has light transmission effects similar to a lens having a parabolic surface.
Here, the second electrode 21 causes behavior of the electric field created by the liquid crystal molecules, making a refractive index of light spatially take the form of a parabolic function. The first electrode 11 corresponds to a lens edge region.
In this case, relatively high voltages are applied to the first electrodes 11 than the second electrode 21. Therefore, as illustrated in FIG. 2, an electric potential difference occurs between the first electrodes 11 and the second electrode 21. In particular, a steep lateral electric field is created around the first electrodes 11. Accordingly, liquid crystals have a slightly distorted distribution rather than a gentle distribution, whereby a refractive index of light cannot exhibit parabolic spatial distribution, or movement of the liquid crystals is excessively sensitive to voltage variation.
The above-described electrically-driven liquid crystal lens of the related art may be realized, without a lens having a parabolic surface, by arranging electrodes on two substrates with liquid crystals interposed therebetween and applying voltages to the electrodes.
The above described electrically-driven liquid crystal lens has the following problems.
Firstly, since the electrodes formed on the lower substrate are positioned at only a part of a lens region, a steep lateral electric field, rather than a gentle electric field, is created between a lens edge region corresponding to the electrode and a lens center region distant from the lens edge region, resulting in a slightly distorted phase of the electrically-driven liquid crystal lens. In particular, in the electrically-driven liquid crystal lens that is driven by a liquid crystal field, since the greater the pitch of lens regions, the smaller the number of electrodes to which high voltages are applied, an insufficient electric field is created between the high voltage electrodes and a substrate opposite these electrodes. Accordingly, it becomes difficult to form the electrically-driven liquid crystal lens having a gentle parabolic lens surface, which has the same effects as an actual lens.
Secondly, when being applied to a large-area display device, the lens center region, which is distant from the lens edge region where the electrode, to which high-voltage is applied, is located, is substantially unaffected by an electric field and has a difficulty in alignment control of liquid crystals by the electric field. As occasion demands, if the alignment control in the lens center region is difficult or impossible, the resulting electrically-driven liquid crystal lens has a discontinuous lens profile and is ineffective as a lens.
Thirdly, since a vertical electric field, created between an electrode to which a high voltage is applied and an electrode formed over the entire surface of a substrate opposite the high voltage electrode, causes a high height, i.e. high sag of the electrically-driven liquid crystal lens and also, the electrically-driven liquid crystal lens requires upper and lower sag margins, a great quantity of liquid crystals may be required to form the entire electrically-driven liquid crystal lens. In particular, since the greater the sag of the electrically-driven liquid crystal lens, the greater the quantity of liquid crystals on a per volume basis, this may result in cost increase and serious deterioration in process efficiency.
Fourthly, a focal distance of the electrically-driven liquid crystal lens is inversely proportional to the sag of the electrically-driven liquid crystal lens. To fabricate an electrically-driven liquid crystal lens having a short focal distance, there is a need for a liquid crystal layer having a large thickness and this becomes a main factor of cost increase. In particular, since the quantity of very expensive liquid crystals increases on a per volume basis as a cell gap increases, there is increasing a demand to reduce the cell gap.
In the electrically-driven liquid crystal lens of the related art, to assure a constant lens profile, the thickness of the liquid crystal layer, i.e. the cell gap must be 30 μm or more and in particular, in the case of a large-area one having a large pitch, the thickness of the liquid crystal layer is further increased. However, an array process for a display panel, such as a liquid crystal panel, forms a cell gap of 10 μm or less, and therefore, is difficult to form the above described high large cell gap of the liquid crystal layer. That is, a current array process forming a liquid crystal panel is difficult to form the liquid crystal layer of the above described electrically-driven liquid crystal lens.
Fifthly, although a Fresnel lens has been proposed in an effort to reduce the above described cell gap, the Fresnel lens has discontinuous surfaces between the respective neighboring sub regions of each lens region having different maximum height points, and thus, causes deterioration in display grade.